ctest_configure: Restore handling of relative CTEST_BINARY_DIRECTORY

In commit e52eada2c2 (cmCTestConfigureCommand: Refactor command line
construction, 2025-01-28, v4.0.0-rc1~60^2~2) we switched from the
`BuildDirectory` setting, which was always an absolute path, to
`CTEST_BINARY_DIRECTORY`.  Convert it to an absolute path too.

Fixes: #27026

Source/CTest/cmCTestConfigureCommand.cxx

@@ -115,11 +115,11 @@ bool cmCTestConfigureCommand::ExecuteConfigure(ConfigureArguments const& args,
     }
 
     configureCommand += " \"-S";
-    configureCommand += sourceDirectory;
+    configureCommand += cmSystemTools::CollapseFullPath(sourceDirectory);
     configureCommand += "\"";
 
     configureCommand += " \"-B";
-    configureCommand += buildDirectory;
+    configureCommand += cmSystemTools::CollapseFullPath(buildDirectory);
     configureCommand += "\"";
   }